1. Field of the Invention
The present invention relates to telecommunication optical cables and in particular it relates to a telecommunication optical cable having a highly reduced diameter, which is particularly suitable for being installed in conduits by a “blown method”.
2. Description of the Related Art
Access and trunk telecommunication networks made by copper wires are being replaced by optical fiber networks in view of their large bandwidth capabilities. As the replacement is submitted to the effective final client requests and is rather expensive for a telecommunication provider, some providers use to arrange a network made only of empty plastic conduits and to install the optical fiber cables in the conduits only when proper requests from the clients have been received. In metropolitan areas, where the available space is usually strict, reduced diameter cables with a medium or high optical potentiality (in terms of number of fibres) is requested for the main network links. A usual optical cable for main network links is generally requested to have a number of fibres not less than 48, typically 72.
A suitable technique to install these optical cables in the conduits is the “blown method”: the optical fibre cable is propelled along the conduit by fluid drag of a gaseous medium, preferably air, blown along the conduit in the desired direction of cable advance. Blowing methods are deemed to be profitable for installing cables in long and short routes due to the lower cost, short time and low tension on the cable. Several features affect the blowing performances of a cable. Such features comprise: inner diameter of the duct, “tortuosity” of the installation path, and cable characteristics (including dimensions, mechanical stiffness and cable weight).
The cable structures mainly employed for blown installation in conduits to form main network links are the Multi Loose Tube (MLT), the Ribbon in Slotted Core (RISC) and the Central Loose Tube (CLT). In turn, CLT cables can be of the “fiber bundle” type or of the “micromodule” type.
A typical MLT cable comprises: a central strength member having a compressive stiffness that is effective to inhibit substantial contraction of the cable and a tensile stiffness that partially or totally contributes to withstand tensile loads without substantially transfer of the tensile loads to the cabled optical fibers; a number of tubes arranged around the central strength member and containing loosely placed optical fibers; a mechanical reinforcing layer, for example a thread made of glass or of an, aramid material arranged around the tubes, if necessary for tensile load cable withstanding; and a protective outer jacket. The tubes containing the optical fibers are typically stranded around the central strength member according to an unidirectional helix or a bidirectional (SZ) helix.
A typical RISC cable includes a cylindrical thermoplastic core extruded around a central strength member and having a plurality of helical slots in its exterior surface. Each slot houses a stack of optical fiber ribbons each having a planar array of optical fibers therein. The slotted core is surrounded by mechanical reinforcing layers and outer sheath as described for MLT cables.
A typical CLT cable comprises: a core tube containing optical fibers, a plastic jacket that surrounds the core tube, and a pair of linearly extending, diametrically opposed dielectric strength rods that are at least partially embedded in the jacket. The strength rods have the same function as the above described strength member of the MLT cable.
Most of the existing terrestrial cables having a medium or high potentiality in terms of number of fibres are not optimized for being blown installed into miniaturized tubes and do not allow to exploit the whole technical and economical advantages of the blown cable installation method.
Thus, the Applicant has perceived a need to provide lightweight and highly reduced-diameter optical cables for being profitably installed in rather small diameter ducts by a blown installation method.
The Applicant has focused on MLT cables with the main object to reduce the diameter thereof.
M. G. Soltis, et al., “Next Generation Loose Tube Cables: Reduce The Size, Not The Performance”, Proceedings of the 49th International Wire & Cable Symposium, pp. 155-163, discusses various design considerations and different steps of development that lead to a reduced size loose tube cable family with performance comparable to larger designs. As far as the tubes containing the fibers are concerned, only problems relating to EFL (excess fiber length) and size have been addressed. Finally, an optical cable having the following characteristics has been presented: number of optical-fibers: 72; number of tubes: 6; fibers per tube: 12; cable diameter: 10.7 mm; and fiber density 0.81 fiber/mm2. According to the Applicant, a similar cable is inadequate for installation through blowing techniques in miniaturized tube infrastructures suitable for metropolitan areas.
P. Gaillard, et al., “Optimization Of Loose Tube Cable Designs: The Next Step”, International Wire & Cable Symposium Proceedings 1998, pp. 106-111, discloses how to reduce cable size and cable installation costs both for MLT and CLT cables. The optimized cable designs, according to this paper, are:                MLT cable: number of fibers: 60; cable diameter: 8.2 mm.        CLT cable: number of fibers: 72; cable diameter: 7.92 mm.        
The nominal cable diameters lead to a cabled fiber density of approximately 1.15 to 1.47 fiber/mm2. In the cable according to P. Gaillard, et al., the tube outer diameter is similar to existing sizes whilst the cable sheath has been reduced as much as possible. Thus, P. Gaillard et al. does not describe nor suggest to increase the fiber density in the loose tubes.
In the attempt of reducing the dimensions of MLT cables, the Applicant has considered the possibility of reducing the diameter of the loose tubes.
The Applicant has carried out some tests and has observed that MLT cables, when the loose tube diameter is reduced, are subject to a number of problems including the following two. First, by reducing the loose tube diameter, the space for the optical fibers is correspondingly, reduced. In other words, the fiber-to-fiber distance becomes lower and the fiber-to-tube distance becomes lower as well. Second, using loose tubes having a reduced diameter with respect to the standard tubes results in reduced Stress Free Window (SFW) safety margins. In this concerns, it should be taken into account that typically the process of tubing the fibers allows to obtain a nominal EFL value with a minimum tolerance of ±0.05% and that this EFL tollerance is considered critical (too large) by the Applicant in order to realize highly miniaturized MLT cables with stable and regular transmission performances.
The Applicant has realized that the reduction of loose tube size results in microbending problems, in turn resulting in higher attenuations of the transmitted signals. The Applicant has conducted several tests and has concluded that a highly miniaturized MLT optical cable can be obtained by reducing the central rod diameter, the outer diameter of loose tubes together with their thickness, and both diameter and thickness of the outer jacket. Whilst such reductions of whole cable diameter and loose tube diameters result in higher fiber densities, the Applicant has realized that the negative effects caused by the increased fiber density within the loose tubes can be eliminated, or at least attenuated, by providing optical fibers having a microbending sensitivity ≦4.0 dB·km−1/g·mm−1 in a temperature range from about −30° C. to +60° C. at 1550 nm and tubes comprising a material having an elasticity, modulus ≧700 MPa.